A field emission ion source has nanostructure materials on at least an emitting edge of the anode electrode. Metal is transferred from a metal reservoir to the emitting edge of the anode, where the metal is transferred to an emitting end of the nanostructure materials and is ionized under an applied electric field. Plural ion sources can be combined to form a field emission ion source device. The numbers of emitting sources are selectable through electric or mechanical switches and different ion extraction potentials can be applied. Various nanostructure materials include: single wall carbon nanotubes and bundles, few-walled carbon nanotubes and bundles, multi-walled carbon nanotubes and bundles, and carbon fiber. Nanostructure-containing material is integrated into the anode by electrophoresis, dielectrophoresis, CVD, screen printing, and mechanical methods. Metal, preferably alkali metal, is transferred into the nanostructure-containing material by one or a combination of following intercalation methods: vapor transport, solution, electrochemical, and solid state reaction.